Power transformers are well known in the art for providing rated voltage and current to electric and electronic devices while isolating those devices from the AC current mains. Ideally, the mains should deliver pure undistorted sinusoidal signals to the primary side of the power transformer. However, in practical applications, this is often not the case. Harmonic components of the fundamental frequency (50 or 60 Hertz) are almost always present, as well as unrelated higher frequency voltages which may be caused by any of a number of sources. For example, spike signals from lightning or the switching of motors, radio frequency signals, digital signals from computer systems, asymmetrical loading of the mains, communication signals, etc., all may contribute to harmonic distortion of the mains power signal.
It is also known that such distortion can, depending on severity, interfere with the optimal functioning of the electrical or electronic equipment connected to the mains, or cause damage to the equipment. In Europe, for instance, three classes have been defined under the recent CE regulations relating to mains distortion. Class A equipment is insensitive to distortion. Class B equipment is influenced to a limited extent by mains distortion without affecting fundamental tasks. Class C equipment ceases functioning under the influence of distortion, but by resetting the equipment, the functioning of the equipment can continue.
Accordingly, the elimination of mains distortion is widely recognized in the art as being highly desirable.
One solution to the problem of eliminating harmonic distortion involves rectifying and buffering the distorted signal to create new pure undistorted sinusoidal voltage signals. This solution is well known in the art of uninterruptable power supplies for use with computers.
Another prior art solution involves the use of resonant transformers which resonate only at the fundamental frequency and therefore attenuate all other frequencies.
Yet another solution involves the creation of "balanced" power lines by means of an external isolation transformer wherein the center tap of the secondary winding is connected to ground, thereby creating two outputs which pass the differential mode distortion in opposite phase.
In all of the foregoing prior art solutions, external elements are required to be added to the power transformer in order to remove differential mode distortion. These solutions introduce additional circuit complexity and attendant costs.
A sample of exemplary prior art patents in the field of transformer means distortion cancellation include:
U.S. Pat. No. 5,640,314 (Glasband et al) PA1 U.S. Pat. No. 5,343,080 (Kammeter) PA1 U.S. Pat. No. 5,206,539 (Kammeter) PA1 U.S. Pat. No. 5,434,455 (Kammeter)